1. Field of the Invention
The present invention relates to an engine exhaust apparatus for a vehicle having a muffler or a silencer which eliminates exhaust gas noises and a method therefor.
2. Description of the Prior Art
In order to reduce sounds of exhaust gases from an engine, a muffler or a silencer is connected to an exhaust passageway. The sounds or noises of exhaust gases generated from the engine are classified roughly into two types. One type is a sound which is generated by the flow in the exhaust passageway. This sound is more pronounced in the high power operating condition of the engine when exhaust pressure becomes high than in the low power operating condition.
The other type is a sound which is generated by echoing in the exhaust passageway. This sound is more pronounced in the low power operating condition of the engine when exhaust pressure remains low than in the high power operating condition of the engine. It is thus noted that either of these two type sounds is more evident in the condition when the other type is decreased or less evident. Accordingly, as a result, sounds caused by exhaust gases cannot be reduced efficiently by using one muffler or silencer, because these two type sounds are incompatible to be erased at the same time.
Therefore, it is a recent trend that a plurality of mufflers or silencers having a shift valve are mounted on the exhaust passageway so as to be operated by a switching valve to open or close at least one of these mufflers or silencers.
For example, Japanese Utility Model Publication (Kokai) No. 58,611/1986 discloses an exhaust system in which one silencer or muffler is provided with two exhaust outlet tubes different in diameter from each other with a shift valve mounted on the large-diameter exhaust outlet tube. The shift valve is operated to close the large-diameter exhaust outlet tube in the region of low engine rotational speed where flow of exhaust gases is sufficiently small to be exhausted merely through the small-diameter exhaust outlet tube. This shift valve is also operated to open the large-diameter exhaust outlet tube so it and the small-diameter exhaust outlet tube are open in the region of high engine rotational speed where flow of exhaust gases is too large to be exhausted only by the small-diameter exhaust outlet tube.
It should be noted, however, in the above apparatus, exhaust gases flow increases extremely in the region of medium engine rotational speed; as a result, noisy sounds occur. Also, the noisy sounds disappear suddenly when the shift valve is opened in the high engine speed region, which gives vehicle drivers a strange feeling.
To suppress such flow sounds, a low pressure exhaust outlet tube may be applied; but, this leads to another problem, namely, echo sounds.